exec(fork_join): don't use __rcvr_ref if it will create env issues

[romintomasetti]

This PR is an attempt for fixing:

• exec::fork_join following a continues_on does not compile #1823

As I understand it, __rcvr_ref::get_env will be instantiated before the compiler sees the definition of _opstate_t::get_env. The relevant compiler error is:

In file included from /workspaces/stdexec/include/exec/fork_join.hpp:18:
/workspaces/stdexec/include/exec/../stdexec/__detail/__receiver_ref.hpp:55:9: error: static assertion failed: get_env() must return the same type as env_of_t<_Rcvr>
   55 |         __same_as<_Env, env_of_t<_Rcvr>>, "get_env() must return the same type as env_of_t<_Rcvr>");
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

...

/workspaces/stdexec/include/exec/../stdexec/__detail/__receiver_ref.hpp:55:9: note: because '__same_as<std::execution::__env::__fwd<std::execution::__sync_wait::__env>, env_of_t<exec::fork_join_impl_t::_opstate_t<std::execution::(anonymous namespace)::__sexpr<std::execution::(lambda at /workspaces/stdexec/include/exec/../stdexec/__detail/__basic_sender.hpp:56:53){}>, std::execution::__tup::__tuple<std::execution::__clsur::__closure<std::execution::__then::then_t, (lambda at /workspaces/stdexec/test/exec/test_fork.cpp:155:30)>, std::execution::__clsur::__closure<std::execution::__then::then_t, (lambda at /workspaces/stdexec/test/exec/test_fork.cpp:155:30)>>, std::execution::__sync_wait::__receiver<>>>>' evaluated to false
   55 |         __same_as<_Env, env_of_t<_Rcvr>>, "get_env() must return the same type as env_of_t<_Rcvr>");
      |         ^
/workspaces/stdexec/include/exec/../stdexec/__detail/__concepts.hpp:64:23: note: because '__is_same(std::execution::__env::__fwd<std::execution::__sync_wait::__env>, std::execution::__env::env<>)' evaluated to false
   64 |   concept __same_as = STDEXEC_IS_SAME(_Ap, _Bp);

meaning that somehow the env_of_t got evaluated to env<>...

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[romintomasetti]

@ericniebler Could you please have a look ?

[ericniebler]

this works, but i have made a separate pr (#1827) with a more targeted fix for the root issue: type deduction in continues_on.